Proton Exchange Membrane Fuel Cell is a power generation device which directly converts chemical energy into electrical energy in an electrochemical manner and is considered to be the most preferred clean and efficient power generation technology in the 21st century. Proton Exchange Membrane (PEM) is a key material for use in Proton Exchange Membrane Fuel Cell (PEMFC).
The perfluorosulfonic acid proton exchange membranes currently used have a good proton conductivity and chemical stability at a relatively low temperature (80° C.) and high humidity. However, they have many shortcomings, such as poor dimensional stability, lowmechanical strength, bad chemical stability and so on. The membrane has different water absorption under different humidity, resulting in different expansion in size, when the membrane transforms under different operation conditions, the size of the membrane changes accordingly. Such case is repeated over and over again then mechanical damage is eventually caused to the proton exchange membrane. Moreover, a large number of substances with strong oxidability, such as hydroxyl radicals and hydrogen peroxide, are produced in a reaction at the positive electrode of a fuel cell, and these substances will attack the non-fluoro groups in the membrane-forming resin molecules, leading to chemical degradation, damage and blistering of the membrane. Finally, when the operating temperature of the perfluorosulfonic acid exchange membrane is higher than 90° C., the proton conductivity of the membrane is decreased sharply due to rapid dehydration of the membrane, thereby decreasing efficiency of the fuel cell greatly. However, high operating temperature can greatly improve the resistance of the fuel cell catalyst to carbon monoxides. In addition, the existing perfluorosulfonic acid membranes have some hydrogen or methanol permeability, especially in a direct methanol fuel cell, permeability of methanol is very high, which becomes a fatal problem. Therefore, how to improve strength of a perfluorosulfonic acid proton exchange membrane, dimensional stability, and efficiency of proton conduction at a high temperature, and to reduce permeability of the working medium and the like becomes a major issue that the fuel cell industry faces.
At present, some methods have been proposed to solve these problems. For example, Japanese Patent No. JP-B-5-75835 enhances strength of a membrane by impregnating a porous media made of polytetrafluoroethylene (PTFE) with a perfluorosulfonic acid resin. However, this PTFE porous medium cannot solve the problems above due to relative softness and insufficient reinforcing effect of the PTFE material. W. L. Gore Co., Ltd developed composite membrane liquid of Gore-Select series by filing Nafion ion conductive liquid with the porous Teflon (U.S. Pat. No. 5,547,551, U.S. Pat. No. 5,635,041, U.S. Pat. No. 5,599,614). This membrane has high proton conductivity and better dimensional stability, however, Teflon has large creep at a high temperature, resulting in performance degradation. Japanese Patent No. JP-B-7-68377 also proposes a method in which a porous media made of polyolefin is filled with a proton exchange resin, but such membrane has insufficient chemical durability and thus there is a problem in long-term stability. Furthermore, due to addition of the porous medium without proton conductivity, the number of proton-conduction pathways is reduced, and proton exchange capability of the membrane is decreased.
Furthermore, Japanese Patent No. JP-A-6-231779 proposes another method for reinforcement by using fluorine resin fibers. The membrane made by this method is an ion exchange membrane which is reinforced through a fluorocarbon polymer reinforcing material in the form of fibrils. However, in this method, it is required to add a relatively large amount of the reinforcing material; in this case, processing of the membrane tends to be difficult, and electrical resistance of the membrane may likely increase.
European Patent No, EP0875524B1 discloses a technology of reinforcing nafion membrane by using glassfiber membrane prepared by applying glassfiber nonwoven technology. Oxides such as silica are also mentioned in this patent. However, non-woven glass fiber cloth is a necessary substrate in this patent, which would greatly limit the application scope.
U.S. Pat. No. 6,692,858 discloses a technology in which a perfluorosulfonic acid resin is reinforced by polytetrafluoroethylene fibers. In this technology, the perfluorosulfonyl fluoride resin and the polytetrafluoroethylene fiber are mixed, extruded, and transformed to prepare a fiber-reinforced perfluorosulfonic acid resin. The method cannot be applied in continuous production due to the time-consuming transformation process.
However, the porous membrane or fiber is only simply mixed with a resin in the above technologies, since the nature of the membrane or fiber differs greatly from the membrane-forming resin, even they are mutually exclusive, it is extremely easy to form gaps between the membrane-forming molecules and reinforcing object, sometimes some pores of the reinforced microporous membrane cannot be filled with the resin. Thus, such a membrane often has high gas permeability, and when the membrane is working in the fuel cell, high permeability tends to result in the energy loss and damage to the cell caused by overheating.